.... there are other rhizome or crown rots caused by other pathogens besides Erwinia, and Sclerotium. These may be caused by Boytrytis, Fusarium or Pythium pathogens. Infection by Pythium sp. seems to be a fairly new problem to rhizomatous iris, but it is a threat to many other ornamentals. Its symptoms of infection are similar to what you've described, especially appearing during moist hot spells.
These diseases respond differently to fungicidal treatments, so my recommendation is to have samples sent to your local county extension agent for analysis. Daconil (chorothalonil) is a shield type fungicide thats effective for foliar treatments, but not so much as a soil drench remedy. Captan seems to have more effectiveness as a soil drench in halting new infections of certain soil borne pathogens. There are many effective fungicidal remedies that are beyond the reach of the average gardener, unfortunately, due to either state restrictions or professional applicator only restriction.
But still, to be absolutely sure, please contact your local county extension agent with a sample. They may send it off to a lab where the pathogen can be isolated and identified
